Umbrella base



FIG. 1 is a perspective view of an umbrella base, shown with environment for ease of disclosure;

FIG. 2 is another perspective view of the umbrella base;

FIG. 3 is an enlarged view of a single hook element of the umbrella base, shown separately for ease of disclosure;

FIG. 4 is an enlarged view of the hook elements secured to the umbrella post of the umbrella base, shown separately for ease of disclosure;

FIG. 5 is a front view of the umbrella base;

FIG. 6 is a rear view thereof;

FIG. 7 is a left side view thereof;

FIG. 8 is a right side view thereof;

FIG. 9 is a top view thereof; and,

FIG. 10 is a bottom view thereof.

The broken lines depict portions of the article and environment that form no part of the claimed design. 

CLAIM The ornamental design for an umbrella base, as shown and described. 